Explanation:
It's the bubble versus the cloud.
NGC 7635, the
Bubble Nebula,
is being pushed out by the
stellar wind
of massive central star BD+602522.
Next door, though, lives a giant
molecular cloud,
visible above to the lower right.
At this place in space,
an irresistible force meets an immovable object in an
interesting way.
The cloud is able to contain the expansion of the bubble gas,
but gets blasted by the hot radiation from the
bubble's central star.
The radiation heats up dense regions of the
molecular cloud causing it to glow.
The Bubble Nebula,
pictured above as a
color negative to help bring up contrast, is about 10 light-years across
and part of a much larger complex of stars and shells.
The Bubble Nebula
can be seen with a small telescope towards the constellation of
Cassiopeia.